You will get a note on the table of the sitting room which talks about a sneak. When you are down in the cellar, there is a secret passage that you can open using the handle of the barrel stands. Once in the secret passage, you can spy on Wade through the slats on a door and you will see him run away and drop something; what he has dropped is the antique key and then you can retrieve it.

I merged your new post about the game with your previous posts about Thornton Hall; if you could post your questions from now on in this one thread, then we can more easily follow along with your progress in the game.